A Simplified Theory of External Occulters for Solar Coronagraphs

TL;DR

This paper develops a simplified analytic theory for multi-vane occulters in solar coronagraphs, enabling better design and performance prediction without extensive simulations.

Contribution

It introduces a sequential plane wave approximation based on Kirchhoff diffraction integral, providing closed-form expressions for multi-vane occulter performance.

Findings

The theory explains observed bright fringing around coronagraphs.

It predicts performance limits of multi-vane occulters.

The approach simplifies design predictions for future instruments.

Abstract

We present a first-principles analytic treatment of modern multi-vane occulters in circular (coronagraph) and linear (heliospheric imager) geometry, develop a simplified theory that is useful for designing and predicting their performance, explain certain visual artifacts, and explore the performance limits of multi-vane occulters. Multi-vane occulters are challenging to design in part because they violate the conditions for both the Fraunhofer and Fresnel approximations to diffraction theory, and new designs have therefore generally required explicit simulation, empirical measurement, "guesstimation", or all three. Starting from the Kirchoff diffraction integral, we develop a "sequential plane wave" approximate analytic theory that is suitable for predicting performance of multi-vane occulters, and use it to derive closed-form expressions for the performance of new designs. We review the fundamental 2-D system of an occulter edge, discuss how it applies to real 3-D systems by extrusion or revolution, present the reason for observed bright quasi-achromatic fringing around coronagraph occulters, develop the sequential plane wave approximation in 2-D and explore its limits, describe the relevance of the 2-D theory to practical 3-D instruments, and discuss implications for multi-vane occulter design in current and future instruments.